A. Technical Field
The present invention relates to a hydrophilic resin, an absorbent article, and acrylic acid for polymerization.
B. Background Art
In recent years, water-absorbent resins, having water-absorbency to the high degree, has been developed as some of hydrophilic resins and are practically used as absorbent articles, such as disposable diapers and sanitary napkins, in combination with fibrous base materials such as cotton, pulp, paper, and sponge. As to the water-absorbent resin, acrylic polymers such as crosslinked partially neutralized polyacrylic acids are industrially most commonly used because of their high water-absorbency, which acrylic polymers are obtained using either one or both of acrylic acid and its salt as the monomer (e.g. JP-A-62-054751).
The above conventional water-absorbent resins have problems in that when preserved for a long time (4 months or longer), they color or discolor to brown or yellow even at room temperature, so the absorbent articles using such water-absorbent resins greatly lose their goods values during preservation.
Concerned with such problems, JP-A-05-086251 regards it as a factor that a very small amount of transition metal in the water-absorbent resin generates radical species to cause unpreferable reactions such as decomposition of the water-absorbent resin and the cleavage of polymer chains, and thus the above document proposed an art in which the coloring with time is prevented by scavenging the transition metal using organic phosphoric acid compounds or their salts.
However, the above prior improving art as disclosed in JP-A-05-086251 has problems as follows.
According to the above prior improving art, as to the water-absorbent resin powder standing alone, its coloring degree (YI) is around 7.8 in the initial stage, but increases up to 35.0xcx9c37.2 when the resin powder is left at 70xc2x0 C. under 65% RH in the closed system for 1 week (Comparative Examples 1xcx9c3 of the ""251 publication), whereas when the organic phosphoric acid compound is added in a ratio of 0.1xcx9c0.63 weight % to the resin powder, the coloring degree (YI) is 20.2xcx9c20.8 after 1 week, so it is assumed in the ""251 publication that the change of the coloring degree (xcex94YI) could be suppressed to 12.4xcx9c13.0 (Examples 1xcx9c3 of the ""251 publication). Indeed the above prior improving art may suppress the coloring with time to some extent, but the addition of the organic phosphoric acid compound not only does complicate the process, but also is not necessarily favorable in view, for example, of the safety. In addition, even Example 2 that provides the most excellent results among the examples of preferred embodiments, as set forth in the above prior improving art, merely results in YI=12.2 and xcex94YI=about 4.4 when the resin is left at 70xc2x0 C. under 65% RH for 1 week, and, naturally, problems of great coloring or discoloring occur when the resin is preserved for a still longer time.
Water-absorbent resins or their products (absorbent articles such as diapers) are internationally traded and, in many cases, preserved for a long time or under high humidity. Therefore, the problems of the coloring often occur. problems and sufficiently bears being practically used. On the basis of such observation results, the present inventors devised the below-mentioned new coloring evaluation method.
Incidentally, some conventional resins, as obtained by reversed-phase suspension polymerization, display the coloring degree (YI) of a little more than 20, but a large amount of hydrophobic organic solvent is used for the reversed-phase suspension polymerization, so the resultant resin has problems on the safety due to the residue of the organic solvent and is therefore not fit for sanitary materials, and further has problems on the cost for the use of the organic solvent. In addition, there are further problems in that the resultant water-absorbent resin comprises spherical fine particles and is therefore difficult to mix with or bind to pulp. and further. is generally insufficient with regard to the crosslinking degree of the surface neighborhood. As a result, the absorption capacity under a load or the liquid permeability is low.
In the above prior improving art, the coloring evaluation is carried out to water-absorbent resins as obtained by polymerization, drying, and pulverization, but there are also problems in that no sufficient study is made about influences to the coloring of factors, such as surface neighborhood crosslinking (as carried out after polymerization and drying) of water-absorbent resin particles, or particle size, shape, or water content of the water-absorbent resin. Generally, examples of the properties which water-absorbent resins should have are as follows: upon contact with aqueous liquids such as body fluids, excellent water absorption amount or speed, the liquid permeability, the gel strength of the swollen gel, the smallness of water-soluble content or monomer residue, the suction power to suck up water from a base material
In recent years, the amount of water-absorbent resin, as used for absorbent articles, tends to increase. The above prior improving art can prevent the coloring if the amount of water-absorbent resin as used is small, but the above prior improving art cannot respond to a large amount of water-absorbent resin. Thus, it is desired to improve the water-absorbent resin itself, namely, to develop a water-absorbent resin that displays extremely little coloring.
The above prior improving art has further problems on the coloring evaluation for the following reason. That is to say, the coloring evaluation in the prior improving art is carried out in the closed system. However, sanitary materials including water-absorbent resins are usually preserved not in an entirely sealed state, but in the at least partially open system (for example, sanitary materials have sewing machine stitches to open their packages), so the coloring evaluation needs to be carried out in the open system.
In the process of diligent study to achieve the below-mentioned objects, the present inventors tried to evaluate the coloring in the open system with regard to water-absorbent resins now on the market. As a result, the coloring degree (YI) was 40xcx9c50 or more. That is to say, the conventional water-absorbent resins displayed the great coloring degree in the open system. Thus, the present inventors confirmed that the coloring evaluation in the closed system according to the above prior improving art had a tendency to display a lower value than that in the open system (for example, coloring degree (YI)=12.2 in Example 2 of the above prior improving art increases to about 23xcx9cabout 30 in the open system evaluation), and further that if YI=20 or less in the open system coloring evaluation, such a water-absorbent resin can solve the above containing aqueous liquids. Among these properties, the absorption actions under a load (e.g. absorption capacity under a load or liquid permeability under a load) are made much of as the fundamental properties which water-absorbent resins should have. There are known methods in which the surface of water-absorbent resins is crosslinked with surface-crosslinking agents for the purpose of obtaining water-absorbent resins of high absorption capacity under a load (EP 668080, U.S. Pat. No. 5,597,873, U.S. Pat. No. 5,422,405, U.S. Pat. No. 5,409,771). In production processes for water-absorbent resins, not only the above surface-crosslinking step but also the drying step of the water-absorbent resin involves heating of the water-absorbent resin. In the above surface-crosslinking step, the polyhydric alcohol is preferable in view of the safety or the resulting properties and is therefore often used, but the polyhydric alcohol makes crosslinking by dehydration and is therefore low reactive. Thus, the surface-crosslinking that involves the use of the polyhydric alcohol needs relatively high temperature or a long time. If the above property improvement or productivity is made much of, the surface-crosslinking or drying step needs heating for a long time and further at high temperature. As a result, thermal degradation or coloring of the water-absorbent resin is unavoidable. Thus, the improvement is needed. Particularly, the water-absorbent resin as surface-crosslinked with surface-crosslinking agents such as polyhydric alcohols has a high tendency to easily become colored during the production or with time thereafter, the above improvement is strongly demanded.
When various absorbent articles such as disposable diapers are produced, it is necessary to combine with a fibrous base material a large quantity of water-absorbent resin which has very high hygroscopicity and of which the main current is powder. Recently, there are increasing problems in that, depending on the environment of working or on weather conditions, powders of the water-absorbent resin cause blocking on the way of the popper or line or adhere to apparatuses, so the absorbent article cannot stably be produced. However, conventional water-absorbent resins as improved upon the above blocking property undergo the deterioration of the absorption capacity under a load and some of the other absorption properties due to the addition of a blocking improvement agent, and further, as to absorbent articles with high resin concentration, the desorption (wet back) of absorbed body fluid tends to increase due to the addition of the above improvement agent. Thus, it is demanded not to add the above improvement agent.
A first object of the present invention is to provide a new hydrophilic resin, such as water-absorbent resin, and a new absorbent article, both of which display still less coloring and discoloring when preserved in the open system for a long time.
A second object of the present invention is to provide a new hydrophilic resin, such as water-absorbent resin, and a new absorbent article, both of which display little deterioration, coloring, or discoloring due to heating at high temperature for a long time in the surface-crosslinking or drying step.
A third object of the present invention is to improve the hygroscopic fluidity (blocking resistance of the water-absorbent resin under high humidity conditions).
In the process of diligent study to achieve the above objects, the present inventors further inferred that the transition metal, which is regarded in JP-A-05-086251 as a factor of the coloring, is not the only one factor of the coloring. Thus, the inventors studied about other factors with encouragement to themselves and with great efforts. The starting materials, as used to produce the water-absorbent resin, usually comprise ten and several kinds of materials, such as bases for neutralization (e.g. sodium hydroxide), crosslinking agents, polymerization initiators, organic solvents, and water, in addition to one or more monomers such as acrylic acid. Among theses materials, acrylic acid is industrially produced commonly by a propylene gas phase oxidation process. Acrylic acid resultant from such a process contains by-products or impurities such as acetic acid, formaldehyde, acrolein, propionic acid, maleic acid, acetone, furfural, and benzaldehyde. Thus, purification is carried out by methods such as solvent extraction and azeotropic dehydration for the purpose of removing those by-products or impurities. However, because acrylic acid easily polymerizes, it is necessary that the purification is carried out in the presence of polymerization inhibitors. Effective polymerization inhibitors are, for example, hydroquinone, hydroquinone monomethyl ether, copper salts, and methylene blue, and particularly, it is common to mainly use hydroquinone, which is the cheapest of the above polymerization inhibitors, and to supplementarily use other expensive polymerization inhibitors (e.g. hydroquinone monomethyl ether). For example, JP-A-10-017524 proposes to use a mixture of an aqueous acetic acid solution and a copper compound as a polymerization inhibitor in the above purification step, but in all the examples of preferred embodiments as set forth in this prior art document, hydroquinone is jointly used with the above mixture.
Because hydroquinone has a high boiling point (boiling point: 285xc2x0 C./730 Torr, melting point: 174xc2x0 C.), it is conventionally thought that even if hydroquinone is used during distillation, hydroquinone does not mingle into vapor of acrylic acid (boiling point: 141xc2x0 C./755 Torr), and therefore is not contained in the finally purified acrylic acid. In fact, on packages of commercially available acrylic acid, it is clearly written that hydroquinone monomethyl ether is contained as the polymerization inhibitor by about 200 ppm, but on none of them, it is clearly written that hydroquinone is contained. Thus, it is conventionally thought that acrylic acid is free from hydroquinone. However, the present inventors found that hydroquinone is contained in purified acrylic acid, although its content is extremely small. One of factors thereof is considered as follows. As is shown by formula (1) below, 
hydroquinone has an equilibrium relation with benzoquinone, and benzoquinone has subliming-property, so hydroquinone becomes benzoquinone because of being heated during distillation on the way of purification to mingle into acrylic acid, and then reverts to benzoquinone. In addition, part of hydroquinone, which mingled into purified acrylic acid, bimolecularly associates in a state of semiquinone (isolation of semiquinone itself is assumed to be impossible) that is an intermediate state in equilibrium between hydroquinone and benzoquinone, thus forming an associated matter (quinhydrone) of formula (2) below (either (A) or (B)): 
The present inventors completed the present invention by finding that this associated matter (quinhydrone) causes the coloring with time. The above-mentioned ten and several kinds of starting materials (e.g. monomers, crosslinking agents, solvents, polymerization initiators) contain various impurities, and the monomer acrylic acid also contains various impurities, and also as to the polymerization inhibitor, many kinds are known. However, the present inventors found that: among the above polymerization inhibitors, surprisingly, an extremely small amount of hydroquinone in acrylic acid causes the coloring with time of the water-absorbent resin; for example, though hydroquinone monomethyl ether is also included in the hydroquinone type polymerization inhibitors similarly to hydroquinone, hydroquinone monomethyl ether has no problem of the coloring with time of the water-absorbent resin, and other impurities (e.g. acrolein, benzaldehyde, acetic add, propionic acid), either, have no problem of the coloring with time of the water-absorbent resin.
Because unneutralized polyacrylic acid is acidic (pH=about 3.0), the ratio for the equilibrium to move in a direction from hydroquinone to benzoquinone is low, and if the hydroquinone content is extremely small, the probability to form the associated matter of formula (2) is low. However, the present inventors found a fact that it is a great cause of the coloring of the water-absorbent resin that, because the water-absorbent resin is required to be neutral (pH=about 6xcx9cabout 8 (as to commercially available ones, pH=about 6.1)) from its usage, the ratio for the equilibrium to move in a direction from hydroquinone to benzoquinone is high, and the probability to form the associated matter of formula (2) is also high. That is to say, it was found that, because the water-absorbent resin is a crosslinked product of a neutral polyacrylic acid, it becomes colored more greatly than water-soluble polyacrylic acids, for example, entirely neutralized polyacrylic acids (neutralization ratio=100 mol %) or unneutralized polyacrylic acids.
For the above reason, the present inventors thought that employing either one or both of the following methods {circle around (1)} and {circle around (2)} is effective for preventing the coloring with time of the water-absorbent resin.
{circle around (1)} A method involving reduction of the ratio for the equilibrium to move in a direction from hydroquinone to benzoquinone to form the associated matter (quinhydrone) of formula (2) in the water-absorbent resin. For achieving this, it is effective to add an quinhydronation inhibitor to the water-absorbent resin.
{circle around (2)} A method in which a water-absorbent resin is produced using acrylic acid with a smaller hydroquinone content than conventional cases, thereby rendering the hydroquinone content in the resultant water-absorbent resin small (actually, as is mentioned above, hydroquinone has the equilibrium relationship with benzoquinone, so the hydroquinone content is the total of hydroquinone and benzoquinone). For achieving this, it is effective to produce a water-absorbent resin by polymerizing a monomer component including a major proportion of either one or both of acrylic acid and its salt which merely have a content of at most 0.20 ppm in total of hydroquinone and benzoquinone.
Then, the present inventors thought that these findings are applicable to the coloring prevention of not only the water-absorbent resin, but also all hydrophilic resins that are produced from acrylic acid.
That is to say, the present invention provides the following constitutions:
(1) A hydrophilic resin, obtained by a process including the step of polymerizing a monomer component including a major proportion of either one or both of acrylic acid and its salt which merely have a content of at most 0.20 ppm in total of hydroquinone and benzoquinone.
(2) A hydrophilic resin according to constitution (1) above, wherein the polymerization is aqueous solution polymerization.
(3) A hydrophilic resin according to constitution (1) or (2) above, obtained by a process further including the step of heat-drying at 100xcx9c300xc2x0 C. within 3 hours after the polymerization.
(4) A hydrophilic resin according to any one of constitutions (1)xcx9c(3) above, comprising particles of the irregular pulverized shape, of which 50 weight % or more have a particle diameter of 300 xcexcm or more.
(5) A hydrophilic resin according to any one of constitutions (1)xcx9c(4) above, which merely has a water content of at most 2 weight %.
(6) A hydrophilic resin according to any one of constitutions (1)xcx9c(5) above, which is a partially neutralized resin.
(7) A hydrophilic resin, comprising a major proportion of an acrylic polymer and a minor proportion of either one or both of hydroquinone and benzoquinone, with the hydrophilic resin further comprising a quinhydronation inhibitor of 10xcx9c1,000,000 times the total weight of hydroquinone and benzoquinone.
(8) A hydrophilic resin according to constitution (7) above, wherein the quinhydronation inhibitor is methylenebisacrylamide.
(9) A hydrophilic resin, comprising a major proportion of an acrylic polymer and merely having a coloring degree (YI) of at most 20 after being left under conditions of the open system, 70xc2x0 C., 65% RH for 1 week.
(10) A hydrophilic resin according to constitution (9) above, which merely displays a change of the coloring degree (YI) by at most 4.
(11) A hydrophilic resin according to constitution (9) or (10) above, wherein the acrylic polymer is a polymer as obtained by a process including the step of aqueous solution polymerization.
(12) A hydrophilic resin according to any one of constitutions (9)xcx9c(11) above, comprising particles of the irregular pulverized shape, of which 50 weight % or more have a particle diameter of 300 xcexcm or more.
(13) A hydrophilic resin according to any one of constitutions (1)xcx9c(12) above, which is a water-absorbent resin.
(14) A hydrophilic resin according to constitution (13) above, which displays pH of 5.5 or less in a physiological salt solution.
(15) A hydrophilic resin according to constitution (13) or (14) above, of which 50 weight % or more have a particle diameter of 300 xcexcm or more.
(16) A hydrophilic resin according to any one of constitutions (13)xcx9c(15) above, of which the surface neighborhood of the particles are crosslinked.
(17) A hydrophilic resin according to constitution (16) above, wherein the crosslinking temperature during the surface-crosslinking is in the range of 100xcx9c250xc2x0 C., and the crosslinking time is within 3 hours.
(18) A hydrophilic resin according to constitution (16) or (17) above, wherein the surface-crosslinking is carried out by a dehydration reaction.
(19) A hydrophilic resin according to constitution (18) above, wherein the crosslinking agent for the dehydration reaction is a polyhydric alcohol.
(20) A hydrophilic resin according to any one of constitutions (13)xcx9c(19) above, which has an absorption capacity of 20 g/g or more for a physiological salt solution under a load of 50 g/cm2.
(21) A hydrophilic resin, which is a water-absorbent resin and is surface-crosslinked or surface-impregnated with a polyhydric alcohol and displays pH of 5.5 or less in a physiological salt solution and has an absorption capacity of 20 g/g or more for a physiological salt solution under a load of 50 g/cm2.
(22) An absorbent article, comprising:
an absorbent layer including the hydrophilic resin as recited in any one of constitutions (13)xcx9c(21) above and a fibrous base material;
a liquid-permeable surface sheet; and
a liquid-impermeable back sheet;
wherein the weight ratio, xcex1, of the hydrophilic resin to the total of the hydrophilic resin and the fibrous base material is 0.3 or more.
(23) Acrylic acid for polymerization, which is acrylic acid as obtained using hydroquinone in its production process and merely has a content of at most 0.20 ppm in total of hydroquinone and benzoquinone.
(24) Acrylic acid for polymerization according to constitution (23) above, which is used for polymerization to give a hydrophilic resin.
(25) Acrylic acid for polymerization according to constitution (24) above, wherein the hydrophilic resin is a water-absorbent resin.
(26) A use of the hydrophilic resin as recited in any one of constitutions (1)xcx9c(21) above under a relative humidity of 50% or higher.
(27) A coloring evaluation method of a hydrophilic resin, comprising the step of judging the coloring degree that the hydrophilic resin displays when left under conditions of the open system, a certain temperature, and a certain humidity.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.